Get the same day of neighboring month.

getSameDayOfNeighboringMonth.ts

/**
 * Get the same day in neighboring month, e.g.
 *
 *   Input: ("2018-10-31", 1)  Output: 2018-11-30
 *
 *   Input: ("2018-10-30", 1)  Output: 2018-11-30
 *
 *   Input: ("2018-01-31", 1)  Output: 2018-02-28
 *
 *   Input: ("2020-01-31", 1)  Output: 2020-02-29
 *
 *   Input: ("2018-01-31", 5)  Output: 2018-06-30
 *
 *   Input: ("2018-01-31", 12)  Output: 2019-01-31
 *
 *   Input: ("2018-01-31", 13)  Output: 2019-02-28
 *
 * @param dt  The date-time expression, a timestamp, or a Date object.
 * @param n   The n(th) month after the `dt`.
 *            Positive integer stands for future month.
 *            Negative integer stands for passed month.
 */
export function getSameDayOfNeighboringMonth(
    dt: number | Date | string,
    n: number = 1
): Date {

    let d = new Date(dt);

    const nextDate = new Date(
        d.getFullYear(),
        d.getMonth() + n,
        d.getDate(),
        d.getHours(),
        d.getMinutes(),
        d.getSeconds(),
        d.getMilliseconds()
    );

    const r = (d.getMonth() + n) % 12;

    if (r < 0) { // Fixed when backward previous years

        if (r + 12 === nextDate.getMonth()) {

            return nextDate;
        }
    }
    else {

        if (r === nextDate.getMonth()) {

            return nextDate;
        }
    }

    return new Date(
        d.getFullYear(),
        d.getMonth() + n + 1,
        0,
        d.getHours(),
        d.getMinutes(),
        d.getSeconds(),
        d.getMilliseconds()
    );
}